The invention relates to matching networks, such as are used in plasma processing systems, which use electronically adjustable inductance elements.
Plasma processing systems are typically used by the electronic device fabrication industry to produce devices such as flat panel displays, integrated circuits, memory chips, etc. In such plasma processing systems, power from a source (e.g. an RF generator) is coupled into a vacuum chamber that contains a particular gas, e.g. argon. Under the appropriate conditions, the power supplied to the chamber produces a plasma in the gas. The plasma is used to perform some phase of a fabrication process, e.g. sputtering a metal target to deposit a thin metal layer onto a substrate, etching material from the surface of the substrate to form device structures or metal pathways interconnecting the devices, reflowing a previously deposited metal layer, etc.
Typically, such plasma systems use some form of impedance matching to maximize the power transfer from the generator to the plasma in the chamber. To see how this is accomplished it is helpful to use a simplified model of the system, as shown in FIG. 1. In this simplified model, an alternating current (AC) generator 10, represented by an ideal current source 10 plus an output impedance of Z.sub.G (16), is connected to an electrical load 12 (i.e., the plasma in the chamber). One terminal of generator 10 and one terminal of load 12 are connected to ground. Maximum power transfer from the generator to the load 12 occurs when the output impedance Z.sub.G of the generator is the complex conjugate of the impedance Z.sub.L of load 12.
Usually, generator and load impedances do not match exactly, and an impedance matching network 18 must be installed between the generator and the load 12, as illustrated in FIG. 2. Often the generator impedance, which is indicated in FIG. 2 as having a value of Z.sub.G, is purely resistive, for most practical purposes. The input impedance of the network 18 with the load connected, as seen looking into the network on input line 20, is Z.sub.IN ; and the output impedance of the network, with the generator connected, as seen looking back into the network on line 22, is Z.sub.OUT. To maximize power transfer from generator 10 to load 12, Z.sub.IN is the complex conjugate of Z.sub.G, and Z.sub.OUT is the complex conjugate of Z.sub.L. The matching network contains one or more adjustable components, e.g. an adjustable inductor, which are tuned, either mechanically or electronically, to produce the Z.sub.IN and Z.sub.OUT which satisfy those conditions.
During a plasma process, the load Z.sub.L changes as the process conditions are altered or even as the process continues. Thus, to maintain a maximum transfer of power from the generator to the plasma, the matching circuit must also change its impedance to satisfy the new matching conditions. Since the changes in process conditions and in the characteristics of the plasma can be quite rapid, it is necessary to have a matching circuit that has a fast response time. For this it is useful to have electrically tunable elements within the matching circuit.
One example of a tunable inductor is described in U.S. Pat. No. 5,392,018, entitled "Electronically Tuned Matching Networks Using Adjustable Inductance Elements and Resonant Tank Circuits", by Ken Collins et al., incorporated herein by reference. The described inductor element shown in FIG. 3 includes a rod-shaped core 30 made of a ferromagnetic material, a primary coil 32 wound around the center region of the core, and two magnetization coils 34 and 36 on either end of the rod core, one on each side of the primary coil. Thus, each of the magnetization coils 34 and 36 has an axis which is collinear with the axis of the primary coil. By using the magnetization coils to vary the magnetic flux density through the core, one is able to modulate the inductance of primary coil. When used in a matching network that is intended for a plasma chamber, the inductor must be water cooled due to the large amount of heat that is generated in the inductor at the power levels that are typically required in such systems.